This volume provides the latest research on circulating tumor cells aimed for cancer researchers, scientists, and molecular oncologists. It presents the basic concepts behind circulating tumor cells (CTCs), metastatic biology, and potential applications as to how CTCs can be used in diagnostic biomarkers. CTCs are cells that have detached from the primary tumor and circulate in the bloodstream. Such cells may become "seeds" for the growth of additional tumors. The field of analysis surrounding cancer metastasis has been steadily growing, and CTCs provide effective biomarkers that can be examined in peripheral blood through a minimally invasive “liquid biopsy� procedure. CTCs offer several exciting applications, not only as markers of disease progression but also as biomarkers of monitoring response to therapy and companion diagnostics for novel anticancer drug development. In recent years there has been rapid growth and worldwide developments on CTCs, which span both the basic sciences and biomedical engineering fields.